Computers serving the City of Newark have been infected by ransomware, according to a report by TAPintoNewark. The computers are unusable and the hacker has demanded 1.7 Bitcoin, the equivalent of $30,000 to unlock the City’s system.

The City’s police department’s computers were unaffected by the RSA-2048 encryption that was installed on Newark computers on Friday.

The malware is usually installed when a user downloads an attachment from a spam or phishing email, according to the report.

Ransomware is a rapidly growing form of extortion, according to a report in the San Diego Union Tribune. There are an estimated 190 forms of the malware that are distributed by emails, bogus advertisements, fake news stories and embedded on websites.

Special Agent Chris Christopherson of the FBI’s San Diego field office told the Union Tribune that ransomware can be avoided or its impact minimized by separating data and exercising skepticism.